Shows a cached image when I try to render
Cegaton's link held the answer:

Came back to a blender file, now not rendering


  The Compositor and Sequence settings are enabled by default, so if there is any data in the compositor or the sequencer blender will
  render from that instead of the scene.
  
  Disable Sequence in Render Settings > Post processing or remove the
  sequencer strips and it will render the scene instead.